Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a substrate or wafer. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. As design rules and process windows continue to shrink in size, inspection systems are required to capture a wider range of physical defects on wafer surfaces while maintaining high throughput.
One such inspection system is a scanning surface inspection system that illuminates and inspects an unpatterned wafer surface. The wafer is scanned under an illumination spot until the desired portion of the wafer surface is inspected. Typically, a laser based illumination source generates illumination light with a Gaussian beam intensity profile. In high-power, laser-based inspection systems, the power density of the incident laser beam is capable of damaging the wafer surface. For inspection systems employing a short-pulsed laser illumination source, substrate damage is primarily related to peak power density. An excessive amount of heat is generated by the interaction of the incident optical radiation with the wafer surface, particularly in areas of incidence subject to incident light with peak power density.
Typically, overall incident beam power is reduced by dumping a portion of the illumination light generated by the illumination source to avoid reaching the thermal damage threshold. In some examples, up to 85% of the beam power generated by the illumination source is dumped to avoid damaging the wafer. In typical bare wafer applications that are shot noise limited, the loss of overall beam power results in a loss of defect detection sensitivity.
Improvements to scanning surface inspection systems are desired to detect defects in the inspection path of an illumination spot on a wafer surface with greater sensitivity while avoiding thermal damage to the wafer surface.